Characterization of antipyrine autoinduction in the rat utilizing a new microsampling technique for serial blood sample collections

Nipecotic Acid: Systemic Availability and Brain Delivery After Nasal Administration of Nipecotic Acid and n-Butyl Nipecotate to Rats

PURPOSE

The purpose of this research was to characterize nipecotic acid pharmacokinetics in blood and brain after intravenous (i.v.) and nasal administration of nipecotic acid and its n-butyl ester.

METHODS

Nipecotic acid and its n-butyl ester were administered to rats i.v. and intranasally (n = 5 rats/drug per route), and nipecotic acid pharmacokinetics in blood were characterized. Nipecotic acid concentration-time profiles were determined in blood by noncompartmental and compartmental methods. Nipecotic acid was also dosed i.v. and its n-butyl ester was dosed by nasal and i.v. routes, and brain levels of nipecotic acid over the subsequent 4 h (n = 5 rats/time point per route) were assessed.

RESULTS

The absolute systemic availability of nipecotic acid after nasal dosing was 14%. After i.v. and nasal dosing of the n-butyl ester, nipecotic acid systemic availability was 97% and 92%, respectively. Both i.v. and nasal administration of the n-butyl ester resulted in a significantly longer terminal half-life and larger mean resident time and volume of distribution for nipecotic acid than was observed after an i.v. nipecotic acid dose. Total brain exposure to nipecotic acid was not significantly different after nasal and i.v. dosing of the n-butyl ester. However, the brain/blood nipecotic acid ratio declined significantly with time after i.v. and nasal dosing of the ester prodrug. Nipecotic acid was not detectable in brain after i.v. dosing of nipecotic acid.

CONCLUSIONS

The use of an ester formulation was crucial to delivering nipecotic acid to the brain. Preliminary evidence strongly suggests ester hydrolysis is rate limiting to nipecotic acid brain delivery. Once nipeoctic acid was formed, it displayed tissue trapping in brain. Parenteral dosing of nipecotic acid esters is unnecessary for systemic or brain delivery of nipecotic acid and possibly other CNS active zwitterion esters.

Altered metabolism of orally administered loxoprofen in human subjects after an oral administration of loxoprofen for three consecutive days followed by a seven-day washout

The effect of pretreatment (i.e., oral administration of loxoprofen for 3 consecutive days followed by a 7-day washout) on the pharmacokinetics and metabolism of the drug was studied in humans. In a control study, a Loxonin tablet (60 mg as loxoprofen anhydrous) was administered orally to 6 healthy male Korean subjects. In a pretreatment study, a Loxonin tablet was administered orally to the subjects once daily for 3 consecutive days. On the 10(th) day, a Loxonin tablet was administered orally to the subjects, and the concentrations of loxoprofen and the trans- and cis-alcohol metabolites in the plasma and urine were measured as a function of time. Using this pretreatment, the area under the curve (AUC) of the trans-alcohol metabolite of loxoprofen in the plasma, but not those of loxoprofen and the cis-alcohol metabolite, was increased (1.5-fold, p < 0.05), leading to increased contribution of the trans-alcohol metabolite to the total urinary recovery of loxoprofen (1.3-fold, p < 0.05). The urinary recovery of total metabolites, which was largely (> 90%) comprised of conjugate metabolites, was also increased as a result of the pretreatment (1.5-fold, p < 0.05). These results indicate that stereoselective reduction to trans-alcohol metabolites as well as the phase II metabolism of loxoprofen may be increased by such a pretreatment in human subjects.

The kinetics of the auto-induction of ifosfamide metabolism during continuous infusion

It has often been reported that the oxazaphosphorines ifosfamide and cyclophosphamide induce their own metabolism. This phenomenon was studied in 21 paediatric patients over 35 courses of therapy. All patients received 9 gm-2 of ifosfamide as a continuous infusion over 72 h. Plasma concentrations of parent drug and of the major metabolite in plasma, 3-dechloroethylifosfamide (3DC) were determined, using a quantitative thin-layer chromatography (TLC) technique. A one-compartment model was fitted simultaneously to both ifosfamide and 3DC data. The model included a time-dependent clearance term, increasing asymptotically from an initial value to a final induced clearance and characterised by a first-order rate constant. A time lag, before induction of clearance began, was determined empirically. Metabolite kinetics were characterised by an elimination rate constant for the metabolite and a composite parameter comprising a formation clearance, proportional to the time-dependent clearance of parent drug, divided by the volume of distribution of the metabolite. Thus, the parameters to estimate were the volume of distribution of parent drug (V), initial clearance (Cli), final clearance (Cls), the rate constant for changing clearance (Kc), the elimination rate constant for the metabolite (Km) and Vm/fm, the metabolite volume of distribution divided by the fractional clearance to 3DC. The model of drug and metabolite kinetics produced a good fit to the data in 22 of 31 courses. In a further 4 courses an auto-inductive model for parent drug alone could be used. In the remaining courses, auto-induction could be demonstrated, but there were insufficient data to fit the model. For some patients this was due to a long time lag (up to 54 h) relative to the infusion time. The time lag varied from 6 to 54 (median, 12)h and values for the other parameters were Cli, 3.27 +/- 2.52 lh-1 m-2, Cls, 7.50 +/- 3.03 lh-1 m-2, V, 22.0 +/- 11.0 1 m-2, Kc, 0.086 +/- 0.074 h-1; Km, 0.159 +/- 0.077 h-1 and Vm/fm, 104 +/- 82 1m-2. The values of Kc correspond to a half-life of change in clearance ranging from 2 to 157 h, although for the majority of the patients the half-life was less than 7 h and a new steady-state level was achieved during the 72 h infusion period. This model provides insight into the time course of enzyme induction during ifosfamide administration, which may continue for up to 10 days in some protocols.(ABSTRACT TRUNCATED AT 400 WORDS)

A pharmacodynamic approach to the estimate of carbamazepine autoinduction

Population-based pharmacokinetic prediction algorithms have been developed for several medications. A fundamental assumption has been that the kinetics remain constant over time. Carbamazepine (CBZ), however, induces its own metabolism in a concentration- and time-dependent manner. A Bayesian estimation program is presented that models the changing catabolic enzyme activity, linearly related to hepatic microsomal enzyme concentration, along with the serum drug concentration. An Emax model is used for enzyme formation with respect to drug concentration: elimination of enzyme activity is modeled as a first-order process. This program was tested in 22 drug-naive outpatients begun on CBZ monotherapy. The 1 week concentrations were used to prospectively predict concentrations at 1 month of therapy and were very close to actual measurements: prediction bias (mean error of prediction) = -0.1 micrograms/mL and precision (median absolute error of prediction) = 1.2 micrograms/mL. Comparison estimates, made by assuming a constant concentration/dose ratio, had bias = 2.6 micrograms/mL (p < 0.001) and precision = 2.2 micrograms/mL (p = 0.01). We conclude that (1) CBZ autoinduction is not complete after 1 week of therapy and (2) the methodology permits accurate estimation of CBZ pharmacokinetics.

Effects of interferon-alpha monotherapy on hepatic drug metabolism in cancer patients

1. The influence of interferon-alpha (IFN alpha) on the clearances of theophylline (TH), antipyrine (AP) and hexobarbitone (HB) was studied in seven cancer patients given IFN alpha as their only treatment. In addition, IFN alpha effects on drug clearance were correlated with changes in serum inflammatory cytokines and acute phase proteins. 2. A 'baseline' study was performed by administering an oral drug 'cocktail' of TH (150 mg), AP (250 mg) and HB (250 mg) with saline injected simultaneously and again 24 h later. One week later, an 'acute' study was performed at the initiation of IFN alpha therapy, 3 x 10(6) units injected with the drug cocktail and again 24 h later. After 2 weeks of IFN alpha treatment three times per week, a 'chronic' study was performed with IFN alpha injected the day prior to, simultaneously with, as well as 24 h after the drug cocktail. 3. Plasma samples were collected over 48 h and the clearances of TH, AP and HB were estimated. Serum samples were collected at various times for the measurement of tumor necrosis factor (TNF), interleukin-1 (IL-1), interleukin-6 (IL-6), C-reactive protein (C-RP) and alpha 1-acid glycoprotein (AGP). 4. IFN alpha caused a 33% decrease in the oral clearance of TH during the chronic study compared with baseline (P < or = 0.05). Although IFN alpha inhibited TH clearance by 16% during the acute study and AP clearance by 20-21% during both acute and chronic studies, these changes did not reach statistical significance. IFN alpha caused minimal changes in HB clearance. There were no chronic effects of IFN alpha on serum cytokines or acute phase proteins. 5. The findings confirm that the most commonly used dose of IFN alpha inhibits the hepatic clearance in humans of some but not all drugs and that this inhibition persists during IFN alpha therapy. Because inhibition was not associated with increases in serum cytokines or acute phase proteins, the mechanism by which IFN alpha inhibits cytochrome P450 activities in vivo does not appear to involve inflammatory mediators such as TNF. IL-1 or IL-6.

Antipyrine as a dialyzable reference to correct differences in efficiency among and within sampling devices during in vivo microdialysis

Antipyrine was investigated as a dialyzable substance that could be used to quantitate relative differences in the efficiency of dialysis among multiple microdialysis probes and by a single probe over time. The contribution of effective membrane surface area to recovery variability was tested by the introduction of air into microdialysis probes. Reduction of effective membrane surface area reduced antipyrine recovery. Dialysates from probes implanted in the jugular vein, brain, and liver of rats receiving antipyrine demonstrated differences in antipyrine concentration among probes within the same rat. These results suggest dissimilar efficiencies of the probes to recover antipyrine, which should be uniformly distributed throughout body water. Dialysates from blood, brain, and liver probes in rats that received both antipyrine and tritiated water (3H2O) showed differences in antipyrine and 3H2O concentrations among probes. Variability of antipyrine and 3H2O concentrations over time within a probe were positively correlated, suggesting that the cause(s) of temporal variability affected both of these markers of body water. Correction of antipyrine tissue/blood ratios, using 3H2O blood/tissue ratios from the same sampling period, reduced the variability in antipyrine tissue/blood ratios, producing ratios closer to the expected value of 1. Differences in probe efficiency contributing to the variability of antipyrine and 3H2O recovery would also be expected to influence the recovery of other substances during microdialysis. The administration of antipyrine during microdialysis experiments is suggested to enable reduction of temporal and site-related differences in substance recovery that are due to differences in probe efficiency. Other methods are necessary to determine the actual extracellular concentration of dialyzed substances and the integrity of the blood-brain barrier.

Fleroxacin pharmacokinetics in patients with liver cirrhosis

In this open-label study, the disposition of fleroxacin in liver disease in 12 healthy male volunteers, 6 male cirrhotics without ascites (group A), and 6 male cirrhotics with ascites (group B) was evaluated. Fleroxacin (400 mg) was administered orally and intravenously to each subject in a random crossover fashion. Fleroxacin was completely absorbed and achieved similar peak concentrations in plasma in all three study groups (P greater than 0.05). The volume of distribution exceeded 1 liter/kg in healthy controls and was not affected by liver impairment (P greater than 0.05). Only group B demonstrated differences in the pharmacokinetic parameters evaluated: the systemic and renal clearances of fleroxacin and the renal clearances and clearances of the two major metabolites of fleroxacin formed, N-demethyl fleroxacin and fleroxacin N-oxide, were significantly lower and the half-lives of the parent drug and its metabolites were significantly longer in group B than in healthy controls and group A (P less than 0.05). The elimination of the two metabolites appeared to be formation rate limited in all three study groups. It was concluded from this study that a 50% reduction in the fleroxacin maintenance dose in patients with liver disease appears justified only in patients with ascites. However, no change in the fleroxacin loading dose is needed in patients with compromised liver function.

Mephenytoin stereoselective elimination in the rat: III. Stereoselective time course of induction during chronic hepatic portal vein administration

The blood concentrations of R- and S-mephenytoin were followed in seven rats over a 5-8 day period during a hepatic portal vein infusion of racemic mephenytoin. In all but two rats, both of the enantiomers achieved an initial steady-state level before a measurable change was observed in their blood concentrations. In each case, the decrease in the initial S-mephenytoin steady state blood level occurred after the change in R-mephenytoin was apparent. During the period of study, the mean +/- SD portal vein clearance of S-mephenytoin increased from 96 +/- 33 ml/h to 450 +/- 160 ml/h. The mean +/- SD portal vein clearance of R-mephenytoin increased from 170 +/- 50 ml/h to 2400 +/- 1600 ml/h. The larger increase in the portal vein clearance for the R-enantiomer resulted in the R/S-mephenytoin clearance ratio over this same time period changing from 1.8 +/- 0.2 to 5.2 +/- 1.8. Attempts to describe the time course of change in the clearance of S- or R-mephenytoin using a previously reported model of induction were unsuccessful. The induction time course did suggest, however, that the rate of induction may be similar for each enantiomer.

Mephenytoin stereoselective elimination in the rat: I. Enantiomeric disposition following intravenous administration

The stereoselective disposition of mephenytoin was characterized after an intravenous bolus dose of racemic mephenytoin to rats being infused with 50% polyethylene glycol 400/50% saline via the jugular and hepatic portal vein. No significant influence on mephenytoin disposition was noted due to the site selected for the administration of the 50% polyethylene glycol 400 solution. The mean (+/- SD) clearance of R- and S-mephenytoin were 171 +/- 58 ml/hr (R) and 110 +/- 37 ml/hr (S), and the mean (+/- SD) volumes of distribution were 325 +/- 75 ml (R) and 359 +/- 72 ml (S). The clearance of R-mephenytoin was significantly larger than the clearance of S-mephenytoin, but this stereoselective difference is of opposite stereochemistry and of much smaller magnitude than the stereoselective difference reported for these enantiomers in man. The difference in the volumes of distribution of R- and S-mephenytoin was not significant.

Quantification of propranolol enantiomers in small blood samples from rats by reversed-phase high-performance liquid chromatography after chiral derivatization

A high-performance liquid chromatographic (HPLC) technique is described for quantification of R(+)- and S(-)-propranolol from 100-microliters rat blood samples. The procedure involves chiral derivatization with tert.-butoxycarbonyl-L-leucine anhydride to form diastereomeric propranolol-L-leucine derivatives which are separated on a reversed-phase HPLC column. The method as previously reported has been modified for assaying serial blood microsamples obtained from the rat for pharmacokinetic studies. An internal standard, cyclopentyldesisopropylpropranolol, has been incorporated into the assay and several derivatization parameters have been altered. Standard curves for both enantiomers were linear over a 60-fold concentration range in 100-microliters samples of whole rat blood (12.5-750 ng/ml; r = 0.9992 for each enantiomer). Inter- and intra-assay variability was less than 12% for each enantiomer at 25 ng/ml. No enantiomeric interference or racemization was observed as a result of the derivatization. No analytical interference was noted from endogenous components in rat blood samples. Preliminary data from two male Sprague-Dawley rats given a 2.0 mg/kg intravenous dose of racemic propranolol revealed differential disposition of the two enantiomers. R(+)-Propranolol achieved higher initial concentration but was eliminated more rapidly than S(-)-propranolol. Terminal half-lives of R(+)- and S(-)-propranolol were 19.23 and 51.95 min, respectively, in one rat, and 14.50 and 52.07 min, respectively, in the other.

Method for chronic portal vein infusion in unrestrained rats

Surgical procedures for the chronic cannulation of the portal and the femoral veins in the rat are described. The surgical procedure for portal vein cannulation is delicate and requires practice, but has proven useful for chronic drug administration. Cannulation of the femoral vein was less involved and cannula patency was extended by proper cannula preparation and placement. This dual cannulation of the rat allowed infusion of racemic mephenytoin into the portal vein and blood collection from the femoral vein for characterizing blood concentrations of the individual isomers. Chronic portal vein drug infusions may provide a useful approach to studies of time-dependent change in hepatic metabolism and stereoselective first-pass drug elimination in unrestrained animals.